Citing anonymous government officials, the newspaper reported that the hackers were connected to a Russian cyber gang and that two other computer systems, at least one connected to a U.S. government agency, were also attacked. The FBI is investigating the case, according to the Wall Street Journal, but the company has flatly denied the story.
"We had no breach of the system and there were no losses, no customer losses, no bank losses," the banking giant said in a statement. "Any allegation that the FBI is working a case at Citigroup involving tens of millions of losses is just not true."
Government sources could not confirm to ABC News whether reports about Citigroup are entirely true but instead said "the truth here is somewhere in the middle."
Citibank was the subject of a significant data breach in early 2009 and has regularly been hit by fraudsters attempting to access account holders' information, a U.S. cybersecurity official told ABC.
Citibank customers' debit and credit card numbers were stolen earlier this year after hackers breached Heartland Payment Systems accounts. The main suspect in that case was allegedly working with accomplices in Russia and Eastern Europe, sources say.
One official told ABC News the alleged compromise at Citibank, reported by the Wall Street Journal, involves a card and bank settlement company similar to Heartland Payment Systems.
Whether or not the Citibank breach occurred, cyber crime is a growing national security concern.
In November, the Department of Justice captured an international hacking group that was planning a $9 million fraud scheme against an Atlanta-based credit card processing company, which is part of the Royal Bank of Scotland. Also last month, the FBI broke up a large computer phishing scheme linked to Egypt, a case the agency called the biggest crime bust in U.S. history.
White House Names Cybersecurity Czar
"Cybersecurity matters to all of us. Protecting the internet is critical to our national security, public safety and our personal privacy and civil liberties. It's also vital to President Obama's efforts to strengthen our country, from the modernization of our health care system to the high-tech job creation central to our economic recovery," counterterrorism czar John Brennan wrote today in an e-mail sent to those who have signed up to receive updates from the White House.
In May, the president stressed the importance of cyber security, calling the threat one of the "most serious economic and national security challenges." At the time, Obama also promised to appoint a cyber czar and unveiled a 60-day cyberspace policy review conducted by the National Security Council and Homeland Security Council.
"It's the great irony of our Information Age -- the very technologies that empower us to create and to build also empower those who would disrupt and destroy," the president said at the White House. "In today's world, acts of terror could come not only from a few extremists in suicide vests but from a few key strokes on the computer -- a weapon of mass disruption. ... This is also a matter of public safety and national security. We count on computer networks to deliver our oil and gas, our power and our water. We rely on them for public transportation and air traffic control. Yet we know that cyber intruders have probed our electrical grid and that in other countries cyber attacks have plunged entire cities into darkness."
ABC News' Devin Dwyer contributed to this report.